1. Field of the Invention
The present invention relates to a receiving circuit and a time piece using the receiving circuit.
2. Description of the Related Art
Recently in some countries, a long-wave standard time electromagnetic wave including a time code is transmitted from a transmitting station. A time piece receives the long-wave standard time electromagnetic wave to acquire the time code and corrects the time using the acquired time code. In Japan, two transmitting stations provide service now to send 40 kHz and 60 kHz long-wave standard time electromagnetic waves respectively. The time code has a frame of 60 seconds per cycle.
In a receiving circuit of a super heterodyne system, an intermediate frequency signal (30 Hz signal) is generated based a received standard time electromagnetic wave signal (40 kHz signal) and the generated intermediate frequency signal is sent to a filter, in which an intermediate frequency component is removed.
FIG. 11 is a view showing an example of a conventional 1; frequency converter/detector circuit and a conventional detector. The standard time electromagnetic wave signal of 40 kHz is amplified by an amplifier 180, and applied to a mixer 182. Meanwhile, the mixer is supplied with an oscillation signal of a local oscillator 181. When an oscillation signal of 39.97 kHz is applied to the mixer 182 from the local oscillator 181, the mixer 182 generates an intermediate frequency signal of 30 Hz. The intermediate frequency signal generated by the mixer 182 is supplied to a band pass filter (BPF) 183, which allows only signal components having frequencies of around 30 Hz to pass through, and amplified by an amplifier 184. The amplified signal is applied to a detector 185. An output signal from the detector 185 is applied to a low pass filter (LPF) 186, whereby an intermediate frequency component is removed from the modulated waveform signal.
JP 2004-80073 A discloses a technique that uses a frequency multiplier circuit to switch a local oscillation signal to be applied to a frequency converter, thereby acquiring an intermediate frequency signal both from a signal of 40 kHz and from a signal of 60 kHz. Further, JP 2004-88341 A discloses a technique that sets a frequency of an oscillation signal of a local oscillator to an intermediate frequency between two frequencies.
As described above with reference to the example shown in FIG. 11, when the intermediate frequency is low enough, for example, when the intermediate frequency is around 30 Hz, the low pass filter having a further lower cut off frequency must be used to decrease the intermediate frequency component of 30 Hz enough to a low level or a high order low pass filter must be used. To adequately integrate the intermediate frequency signal of 30 Hz, a filter having a considerably large time constant must be used. For example, when a filter having the cut off frequency of 5 Hz is used to allow a signal of 5 Hz to pass through, a signal component of 30 Hz is decreased only to degree of 7 dB, and considerable levels of the signal component remains.
Usage of a filter of a low cut off frequency brings a problem that degrades signal quality. Meanwhile, usage of a high order filter brings a problem that a large scale of circuit must be used and the circuit is made more complicated.